1. Field of the Invention
The present invention relates to a photographic printing apparatus and more particularly to a printing apparatus which uses a printing light source unit having a plurality of illumination lamps which are exchangeable when one of them breaks.
2. Description of the Prior Art
For better printed color balance and density, general photographic printing apparatus measures an original such as a negative film to detect light passed through the original in order to obtain the large area transmittance density of the original. The large area transmittance density is an approximate density which is expressed as the logarithm of the reciprocal of a value equivalent to the amount of light passed through the whole area of the original. According to the density of the original, the proportion of the three color components, namely red, green and blue components, of the printing light is adjusted to make an exposure with will produce a print of good color balance and density. For calculating proper exposure for each color, the following equation is conventionally used: EQU Log Ti=Log T.sub.Ni +C.sub.j (D.sub.Nj -D.sub.NNj) (1)
where
T.sub.i : exposure time;
T.sub.Ni : constant (exposure time for a normal density standard negative);
i: any one of red, green and blue;
C.sub.j : slope control coefficient:
D.sub.Nj : density of printing negative;
D.sub.NNj : density of a normal density standard negative; and
j: any one of cyan, magenta and yellow.
The constant T.sub.Ni and coefficient C.sub.j are determined upon setting printing conditions suitable for various densities of originals. In particular, the constant T.sub.Ni is determined depending upon an exposure time which is suitable for making a print with the most suitable gray density from standard negative having a normal density D.sub.NNj. On the other hand, the slope control coefficient C.sub.j is determined depending upon exposure times which are suitable for under and over exposed negatives, respectively, to make prints with the same gray density as the print made from the normal density standard negative.
Conventional photographic printers have only one illumination lamp for printing negatives. Therefore, upon breakage of the illumination lamp during printing, the printing operation should be suspended in order to replace the broken illumination lamp with a new one which is usually of the same kind as the broken one. Although the two lamps are of the same kind, there is a somewhat considerable difference in optical characteristics such as luminance color between the two. The printing and condition setting, which has been made for the broken illumination lamp, has to be conducted again for the new illumination lamp. The printing condition setting is quite troublesome and time consuming because of, as was described previously, the necessity of trial prints.